Method of preparation of single crystal films

ABSTRACT

A method of preparation of a single-crystal thin film of a rare earth mixed oxide having a garnet structure of the formula T3M5O12 where M respresents iron and T represents Yttrium. A single-crystal substrate is placed in an aqueous solution which is capable of liberating the constituents of the single-crystal film in situ; the solution is brought to a high temperature in order to dissolve the constituents of the single-crystal film; and these constituents are then deposited on the single-crystal substrate by recrystallization.

Ferrand et al.

Commissariat a lEnergie Atomique, Paris, France Filed: May 22, 1972Appl. No.: 255,549

Assignee:

Foreign Application Priority Data May 25, 1971 France 71.18889 US. Cl117/235, 117/113, 423/263, 423/594 Int. Cl. H011 10/02 Field of Search117/235-240, 117/113, 65.2; 423/594, 263

References Cited UNITED STATES PATENTS 7/1962 Stoller et al. 117/235Nov. 19, 1974 3,100,158 8/1963 Lemaire et a1 ll7/235 X 3,404,026 10/1968Skudera et al. 117/235 UX 3,429,740 2/1969 Mee 117/235 X 3,486,93712/1969 Linares 117/235 X 3,573,099 3/1971 Moore et al. 117/235 X3,607,390 9/1971 Comstock 117/235 X 3,697,320 10/1972 Hiskes 117/235 XPrimary ExaminerWilliam D. Martin Assistant Examiner-Bernard D. PianaltoAttorney, Agent, or FirmCameron, Kerkam, Sutton, Stowell & Stowell [5 7]ABSTRACT A method of preparation of a single-crystal thin film of a rareearth mixed oxide having a garnet structure of the formula T M O where Mrespresents iron and T represents Yttrium. A single-crystal substrate isplaced in an aqueous solution which is capable of liberating theconstituents of the single-crystal film in situ; the solution is broughtto a high temperature in order to dissolve the constituents of thesingle-crystal film; and these constituents are then deposited on thesingle-crystal substrate by recrystallization.

10 Claims, No Drawings METHOD OF PREPARATION OF SINGLE CRYSTAL FILMSThis invention relates to a method of preparation of single-crystalfilms, especially thin films of mixed oxides having a garnet structure.

The invention applies more particularly to the preparation of films ofrare-earth single-crystals, synthetic double oxides corresponding to theformula T3M5O12 in which is oxygen, M is iron which may be replacedeither wholly or in part by aluminum, gallium or one of the transitionelements having an atomic number within the range of 21 to 28 inclusiveand in which T represents yttrium or one of the rare-earth elementshaving an atomic number within the range of 62 to 71, or alternatively amixture of said rare-earth elements with each other or with yttrium.

The single-crystal films defined in the foregoing exhibit advantageousmagnetic properties which permit their use in particular in theultra-high frequency range (as resonant insulators, transducers,surface-wave amplifiers). These films can also be employed in logic andstorage devices of the domain-displacement type.

One of the methods employed up to the present time for obtaining filmsof rare-earth single-crystals consists in growing a single crystal T MO, on a seed of T M O by hydrothermal synthesis. This method was carriedout in practice as follows: the T M O was dissolved in thepolycrystalline form in a 20 M sodium hydroxide solution, in the bottomportion of an autoclave which was heated to 400C; the single crystal wasthen recrystallized on a seed of T M O in the top portion of theautoclave which was heated to 375C.

Another conventional method of formation of a single-crystal filmconsisted in growing a single crystal of T M O, on a seed of GdGaG(gadolinium-galliumgarnet). A solution of 20 M NaOH or 20 M KOH at atemperature in the vicinity of 400C was employed as solvent. But itproved difficult to apply this method to the preparation of thin filmssince the seed was frequently attacked by the solvent. There was alsoemployed as solvent a percent solution of NH Cl at temperatures in thevicinity of 550C. However, the dissolution of the polycrystal T M O inthe ammonium chloride was a very slow process, the seed of GdGaG wasattacked by NH Cl and the single-crystal deposit was not uniform.

The method according to the invention overcomes the disadvantagesrecalled in the foregoing, especially insofar as it permits rapiddissolution of the constituents of the single crystal and consequentlyresults in thin and homogeneous single-crystal films. One of theadvantages ofthc invention lies in the fact that a singlecrystal filmcan be deposited on a single-crystal substrate having a differentchemical composition.

The method under consideration is primarily distinguished by the factthat a single-crystal substrate is placed in an aqueous solution whichcontains compounds having a base of constituents destined to form thesingle-crystal film and which is capable ofliberating said constituentsin situ, said solution is brought to a high temperature so as todissolve said constituents, and said constituents are then deposited byrecrystallization on said single-crystal substrate.

In a preferred mode of execution of the invention, thin single crystalfilms of a rare-earth mixed oxide having a garnet structure, andcorresponding to the formula T M O as hereinabove defined. are depositedon a single-crystal substrate consisting ofgadoliniumgallium-garnet-(GdGaG).

It will be readily understood that this composition is not intended toimply any limitation of the invention and the method can be employed forany other deposition of a single-crystal film of mixed oxides having agarnet structure on a predetermined single-crystal substrate. The methodcan also be applied to the preparation of single-crystal thin films ofdifferent magnetic compounds of garnets such as, for example, theorthoferrites of rare earths or the spinel ferrites.

The method according to the invention is carried out in an autoclave ofheat-resisting steel lined with platinum, gold or silver. This autoclaveis hermetically sealed by means of a system of gaskets of the Bridgmantype and connected to a manometer by a highpressure flexible capillarywire. The autoclave is placed in a furnace having two separate heatingzones which are regulated independently in order that it may thus bepossible to control the temperature difference between the top and thebottom of the autoclave.

The method is carried into effect as follows:

The constituents which are intended to yield in situ the mixed oxidehaving a garnet structure by double decomposition are placed in anaqueous solution in the bottom of the autoclave which is the hotportion,

The single-crystal substrate is placed in the top of the autoclave whichis the cold portion,

The aqueous solution is brought to a high temperature and high pressurein such manner as to decompose and dissolve the constituents of themixed oxide, then to transport them into the cold portion of theautoclave in which they crystallize on the single-crystal substrate.

The temperature difference existing within the autoclave gives rise toconvection currents which facilitate the transport of material from thehot portion to the cold portion.

In accordance with one advantageous feature of the method forming thesubject of this invention, the aqueous solution is constituted by asolution of a mixed salt of a metal and of an alkali metal and ahydroxide or a salt of rare earth or of yttrium. For example, it ispossible to employ NaFeO or KFeO and Y (Oi-U Double decompositionresults in formation on the one hand of sodium hydroxide or of potassiumhaving a low concentration which acts as transporting and mineralizingagent and on the other hand of single-crystal film constituents whichwill be deposited on the single-crystal substrate.

In accordance with another feature of the method which forms the subjectof the invention, the aqueous solution is constituted by a solution of amixed salt of rare earth or of yttrium and of an alkali metal and ahydroxide or a salt of a metal.

For example, it is possible to carry out a double decomposition reactionbetween NaFeO and an yttrium salt such as, for example, YCI 6 H O; Y(N09 The salts which are formed, namely NaCl, NaNO accordingly act astransporting and mineralizing agents. It is also possible to carry out adouble decomposition reaction between YNaO or YKO and newly precipitatedFe (OH) or a ferric salt such as, for example, FeCl Fe (N09 Fe (SO Inthe method according to the invention, the operational temperature iswithin the range of 450C to 550C. A temperature difference of 20C to 70Cis maintained between the bottom portion and the top portion of theautoclave.

The coefficient of filling in the autoclave is 0.4 to 0.6.

One example of application of the method according to the invention isgiven hereinafter without any implied limitation.

EXAMPLE Use is made of an autoclave having a useful volume of 130 cm andlined with silver.

There are placed at the bottom of said autoclave 6 g of NaFeO 2.5 g ofY(OH) and a sodium hydroxide solution consisting of 2.6 g of causticsoda in 60 cm of water.

There is then fitted in position a platinum frame provided with acentral screen having an opening of 20 percent.

At the top of said frame, there is placed a seed of Gd Ga 0. which iscut at right angles to the direction (1, l,

The autoclave is closed and placed in a furnace having two temperaturezones in order that the bottom portion of the autoclave should be at atemperature of 500C and that the top portion should be at a temperatureof 450C.

The pressure is then 600 bars. The seeding process is carried out for aperiod of 24 hours.

At the end of this period, the autoclave is cooled rapidly and there isthus obtained a thin single-crystal film of Y Fe O, having a thicknessof 2.5 11..

What we claim is:

l. A method of preparation of a single-crystal thin film of a rare earthmixed oxide having a garnet structure of the formula T M O where Mrepresents iron and T represents yttrium, the steps of placing asinglecrystal substrate in an aqueous solution in an autoclave whichcontains compounds having a base of Constituents to form thesingle-crystal film by liberating said constituents in situ by doubledecomposition, bringing said solution to a high temperature and highpressure to dissolve said constituents and then depositing saidconstituents by recrystallization on said single-crystal substrate, atemperature difference within the range of 20C to C being maintainedbetween dissolution and deposition of the constituents.

2. A method according to claim 1, the coefficient of filling of theautoclave being between 0.4 to 0.6.

3. A method according to claim 1, the iron being at least partiallyreplaced by a substance selected from the group consisting of aluminum,gallium and the transition elements having an atomic number within therange of 21 to 28 inclusive.

4. A method according to claim 1, T being a substance selected from thegroup consisting of the rare earth elements having an atomic numberwithin the range of 62 to 71.

5. A method according to claim 1, the single-crystal substrate having achemical composition different from that of the single-crystal film.

6. A method according to claim 1, the single-crystal substrate beinggadolinium-gallium-garnet.

7. A method according to claim 1, the aqueous solution being a solutionof a mixed salt of a metal and of an alkali metal and a hydroxide or asalt of rare earth or of yttrium.

8. A method according to claim 1, the aqueous solution being a solutionof a mixed salt of rare earth or of yttrium and of an alkali metal and ahydroxide or a salt of a metal.

9. A method a-cording to claim 8, wherein the solution contains NaFeOand YCl 6H O.

10. A method according to claim 1, the high temperature being between400C. and 550C.

1. A METHOD OF PREPARATION OF A SINGLE-CRYSTAL THIN FILM OF A RARE EARTHMIXED OXIDE HAVING A GARNET STRUCTURE OF THE FORMULA T3M5O12 WHERE MREPRESENTS IRON AND T REPRESENTS YTTRIUM, THE STEPS OF PLACING ASINGLE-CRYSTAL SUBSTRATE IN AN AQUEOUS SOLTUION IN AN AUTOCLAVE WHICHCONTAINS COMPOUND HAVING A BASE OF CONSTITUENTS TO FORM THESINGLE-CRYSTAL FILM BY LIBERATING SAID CONSTITUENTS IN SITU BY DOUBLEDECOMPOSITION, BRINGING SAID SOLUTION TO A HIGH TEMPERATURE AND HIGHPRESSURE TO DISSOLVE SAID CONSTITUENTS AND THEN DEPOSITING SAIDCONSTITUENTS BY RECRYSTALLIZATION ON SAID SINGLE-CRYSTAL SUBSTRATE, ATEMPERATURE DIFFERENCE WITHIN THE RANGE OF 20*C TO 70*C BEING MAINTANEDBETWEEN DISSOLUTION AND DEPOSITION OF THE CONSTITUENTS.
 2. A methodaccording to claim 1, the coefficient of filling of the autoclave beingbetween 0.4 to 0.6.
 3. A method according to claim 1, the iron being atleast partially replaced by a substance selected from the groupconsisting of aluminum, gallium and the transition elements having anatomic number within the range of 21 to 28 inclusive.
 4. A methodaccording to claim 1, T being a substance selected from the groupconsisting of the rare earth elements having an atomic number within therange of 62 to
 71. 5. A method according to claim 1, the single-crystalsubstrate having a chemical composition different from that of thesingle-crystal film.
 6. A method according to claim 1, thesingle-crystal substrate being gadolinium-gallium-garnet.
 7. A methodaccording to claim 1, the aqueous solution being a solution of a mixedsalt of a metal and of an alkali metal and a hyDroxide or a salt of rareearth or of yttrium.
 8. A method according to claim 1, the aqueoussolution being a solution of a mixed salt of rare earth or of yttriumand of an alkali metal and a hydroxide or a salt of a metal.
 9. A methoda-cording to claim 8, wherein the solution contains NaFeO2 and YCl2,6H2O.
 10. A method according to claim 1, the high temperature beingbetween 400*C. and 550*C.